The invention relates to a DC/DC converter, comprising inductive energy storage means, switching means and control means, wherein the control means are arranged for selectively operating the switching means for providing electrical energy from the energy storage means to an output of the DC/DC converter in both a Pulse Width Modulation (PWM) mode and a Pulse Frequency Modulation (PFM) mode switching cycle.
A DC/DC converter of this type is know from U.S. Pat. 5,568,044.
Depending on the load at the output, the known converter selects a particular conversion mode. That is, at a relatively low output load the PFM or discontinuous mode is selected, whereas for a relatively high output load, up to its maximum, the converter operates in the PWM or continuous mode. This, in order to control the output voltage of the DC/DC converter with a high efficiency over a large output power range.
In a single-output DC/DC converter operated in PWM mode, the output power is controlled by its duty cycle, which is the ratio of the length of a first phase of the switching cycle wherein energy is stored in the energy storage means and the total length of the switching cycle. This is a linear relationship, such that an increase of the duty cycle results in an increase of the energy to be stored and, accordingly a higher output power to be delivered at the output. The duty cycle is determined by the ratio of the input voltage and the output voltage of the DC/DC converter.
In the PFM mode a constant output voltage is maintained by skipping switching cycles while the output voltage exceeds its nominal value.
In a multi-output DC/DC converter the output voltage of each of the outputs has to be controlled separately. In a PWM mode, for example, this results in different values for the duty cycle of each output, because the ratio of the input voltage and the output voltage differs for each output. However, the duty cycle is also related to the amount of energy to be stored and the total energy to be provided at the output of the converter. Due to the different output voltages this relation is not linear. Accordingly, the control principles for a single-output DC/DC converter cannot be used for multi-output DC/DC converters.
It is an object of the present invention to propose a DC/DC converter, providing a correct distribution of the stored energy to the different outputs, either in a PWM mode or a PFM mode dependent on the output power required by the load at a particular output.
This object is solved by the present invention in a DC/DC converter of the type mentioned above, in that the control means are arranged for controlling multiple outputs of the DC/DC converter by providing a sequence of a number of separate switching cycles for each output, and for establishing a main output among the multiple outputs requiring the highest amount of energy, wherein the main output in PWM mode determines the total amount of energy to be supplied in a sequence, and the number of switching cycles of an output in PWM mode is determined by part of the total amount of energy to be supplied at this output.
In accordance with the present invention, with each output of the multi-output DC/DC converter a number of switching cycles in a sequence of switching cycles is associated. The total amount of energy to be supplied in a sequence is, in accordance with the invention, determined by a so-called main output in PWM mode, which is the output of the converter requiring the highest amount of energy. Then, the number of switching cycles of any other of the outputs in PWM mode is determined from which part of the total amount of energy has to be supplied to a particular output.
In order to determine which of the outputs is active during a sequence, in a further embodiment of the invention, the control means are arranged for determining of each output a so-called ratio, being the number of switching cycles for an output in a sequence divided by the total number of switching cycles of the sequence, such that an active output is selected based on the ratio and output voltage of this output.
In PFM mode, a new switching cycle is started if the output voltage drops below a reference voltage and if the ratio determines whether the output is to be switched in the sequence.
Accordingly, if the output voltage for an output in PFM mode has not dropped below a reference voltage, no new switching cycle will be started in the particular sequence. In such a case, in the sequence, the output can be replaced by one of the other outputs in PWM mode, for example.
In the case of a main output in PWM mode, in a further embodiment of the DC/DC converter according to the invention, the control means are arranged such that the output voltage of the main output controls the ratio of the main output and the total energy provided in a sequence, and in that for an other output in PWM mode the output voltage thereof controls the ratio of this output and part of the total energy to be provided at this output in a sequence.
Thus, for an output in PWM mode the ratio of an output determines when a new switching cycle is started for this output. When all outputs are in PWM mode the ratio of an output is equal to the number of switching cycles of the output in the sequence. If several outputs operate in a PWM mode, the main output is the output having the highest ratio.
In a practical embodiment of the DC/DC converter according to the invention, each output comprises separate control means, having a duty cycle output, a ratio output and a mode output, timer means connected to the duty cycle output of the control means for controlling the switching means, means for establishing a main output and an active output, which means being connected to the ratio and the mode output of each of the control means and having an active output connected to the timer means and to an input each of the control means and a main output connected to a further input of the control means, and comparator means for comparing of each output its output voltage with a reference voltage, an output of the comparator means connect to an input of an associated control means.
In this embodiment of the DC/DC converter according to the invention, the output voltages of the several outputs are compared with reference voltages, wherein the result of the comparison is used to determine the next action for the momentary active output. Each output has its own controller for determining its mode (PWM of PFM), duty cycle and ratio. The main output and the active output are determined by the duty cycle and ratio of all outputs. The timer means drive the switches according to the active output, mode and duty cycle of a particular output, i.e. control means.
The invention may be practised with a plurality of DC/DC converter designs, such as multi-output DC/DC up converters, multi-output DC/DC down converters, multi-output DC/DC up/down converters, multi-output DC/DC inverting converters, multi-output DC/DC converters with positive and negative output and multi-output DC/DC up/down converters with multiple positive and negative outputs, for example.
The invention also relates to a power supply comprising a DC/DC converter disclosed above, and arranged for receiving an input voltage at input terminals and for providing controlled output voltages at multiple output terminals.
The DC/DC converter according to the invention is of particular advantage if applied in an electronic appliance, such as, but not limited to, a portable electronic appliance.